Add Base URL to Amplitude integration

[khvn26]

Is your feature request related to a problem? Please describe.

Amplitude clients with EU data residency now need to call a separate base URL.
As per the docs:

For EU data residency, the project must be set up inside Amplitude EU. Replace the standard endpoint https://api2.amplitude.com/2/httpapi with the EU residency endpoint https://api.eu.amplitude.com/2/httpapi in all examples to ensure proper data residency.

Flagsmith currently hard-codes the Amplitude base URL.

Describe the solution you'd like

1. Expose the Base URL field in the integration create UI.
2. Migrate existing Amplitude integrations to store the default base URL in the configuration model.
3. Update the Core and Edge integration code to use the stored base URL.

Describe alternatives you've considered

N/A

Additional context

N/A